Pressure sensing is one of the most established areas of sensor technology. One specialised application of pressure sensors is hydrostatic tank gauging (HTG). HTG is a pressure-based tank gauging system that uses a combination of pressure and temperature measurements to provide a totally automated, multiple measurement system for liquid inventory measurements.
Using pressure sensors for HTG systems is an emerging way to accurately gauge liquid inventory as well as to monitor transfers in tank farms and similar multi-tank storage facilities. Increasingly, HTG systems are also employed for storage tank leak detection. The interest in pursuing better leak detection and prevention methods is prompted by concerns for environmental protection, coupled with increasingly stringent legislation and regulation.
Traditional HTG installations involve disrupting the integrity of the tank wall in three or more places to mount multiple pressure and temperature sensors. Each sensor is a complex combination of electrical and mechanical components. Microelectromechanical Systems (MEMS) technology offers a means of eliminating the need for multiple sensors as it allows on-chip integration of pressure and temperature transducers.
While there is potential for combining various sensors and signal conditioning circuit into one microelectromechanical system, silicon micromachined capacitive pressure sensors such as those described in U.S. Pat. Nos. 6,631,645, 6,051,853, 6,122,973 and 6,595,064 are not suitable for hydrostatic measurements. The reason for this is that pressure sensors for HTG systems must be able to withstand the large pressure inside the tanks, and at the same time be sensitive to the relatively small pressure changes brought about by variations in the fluid.
In typical micro-capacitive pressure devices, a flexible diaphragm serves as one electrode of a capacitor, whereas the other electrode is located on a substrate beneath it. As the diaphragm deflects in response to the applied pressure, the average gap between the electrodes changes, leading to a change in the capacitance. For capacitive pressure sensors to operate in a high pressure environment, the movable plate must be thick. The trade-off introduced by the use of a thick diaphragm is typically low sensitivity to small changes in pressure. Consequently, a parallel plate capacitive pressure sensor would not be capable of detecting the relatively small pressure variations in a high pressure environment.
Hence, it is with the knowledge of the above concerns and restrictions that the present invention has been made.